Developer Thom Haslam has released HarmonyKeen, a harmony assistant that writes parts around your melodies using the music theory voice-leading rules.

You give it a monophonic MIDI melody and it writes harmony parts around it, working out the voice leading. Everything it writes is editable MIDI, so the parts drive whatever instruments you already have.

Styles run from a single added line through close vocal harmony and four-part writing to fuller arrangements.

It runs on music theory rather than a neural network. It reads the whole melody first, finds the phrases and cadences, picks a chord sequence that fits, then writes the voices. The same input and settings give the same result every time.

The HarmonyKeen site features a wide selection of examples, demonstrating the range of harmony options that it supports.

Pricing and Availability:

HarmonyKeen is available now as a standalone app plus VST3 and Audio Unit, bundled on one license, with an intro price of $59 USD (normally $79).